U.S. patent number 4,921,331 [Application Number 07/138,108] was granted by the patent office on 1990-05-01 for multi-layered mirror.
This patent grant is currently assigned to Hoya Corporation. Invention is credited to Yuji Nakajima.
United States Patent |
4,921,331 |
Nakajima |
* May 1, 1990 |
Multi-layered mirror
Abstract
An improved mirror of a type especially advantageous for use as
a rearview mirror for automotive applications. The inventive mirror
employs a dielectric multi-layered coating formed on the front side
of a glass or plastic transparent substract, the back side of which
is provided with a light-absorbing layer. The dielectric coating is
composed of three to six dielectric layers laminated one on top of
another and having alternately differing refractive indices. At
least one of the laminated dielectric layers has an optical
thickness of .lambda..sub.0 /2, where .lambda..sub.0 is a
wavelength of light used as a reference for design purposes and
which is in a range of 500 to 580 nm for an incident angle of
0.degree., with each of the remaining dielectric layers having a
thickness of .lambda..sub.0 /4.
Inventors: |
Nakajima; Yuji (Tokyo,
JP) |
Assignee: |
Hoya Corporation (Tokyo,
JP)
|
[*] Notice: |
The portion of the term of this patent
subsequent to February 21, 2006 has been disclaimed. |
Family
ID: |
17989034 |
Appl.
No.: |
07/138,108 |
Filed: |
December 28, 1987 |
Foreign Application Priority Data
|
|
|
|
|
Dec 27, 1986 [JP] |
|
|
61-309112 |
|
Current U.S.
Class: |
359/584; 359/603;
359/884 |
Current CPC
Class: |
A47G
1/02 (20130101); B60R 1/083 (20130101); C03C
17/3417 (20130101); G02B 5/0825 (20130101); G02B
5/0833 (20130101); G02B 5/26 (20130101) |
Current International
Class: |
A47G
1/00 (20060101); A47G 1/02 (20060101); B60R
1/08 (20060101); C03C 17/34 (20060101); G02B
1/10 (20060101); G02B 5/08 (20060101); G02B
005/08 (); G02B 005/28 () |
Field of
Search: |
;350/164,600,166,642,278,276R,276SL ;428/333 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Arnold; Bruce Y.
Assistant Examiner: Callaghan; Terry S.
Attorney, Agent or Firm: Sughrue, Mion, Zinn, Macpeak &
Seas
Claims
I claim:
1. A multi-layered mirror comprising:
a substrate made of one of glass and transparent plastic;
a dielectric multi-layered coating disposed on the front side of
said substrate; and
a light-absorbing layer formed on the backside of said
substrate,
said dielectric multi-layered coating comprising three to six
dielectric layers laminated one on top of another and having
alternately differing refractive indices, one of said dielectric
layers being one of a single high-refractive index layer and single
low-refractive index layer having an optical thickness of
.lambda..sub.0 /2, where .lambda..sub.0 is a wavelength of light
used as a reference measurement for design purposes, and each of
the remaining layers has an optical thickness of .lambda..sub.0
/4.
2. A multi-layered mirror according to claim 1, wherein said three
to six layers are selected from L, H, 2L and 2H layers, wherein
said H layer is a layer made of a material having a high refractive
index of 1.9 to 2.4 and an optical thickness of .lambda..sub.0 /4,
said 2H layer is made of a material having a high refractive index
of 1.9 to 2.4 and an optical thickness of .lambda..sub.0 /2, said L
layer is made of a material having a low refractive index of 1.3 to
1.8 and an optical thickness of .lambda..sub.0 /4, and said 2L
layer is made of a material having a low refractive index of 1.3 to
1.8 and an optical thickness of .lambda..sub.0 /2.
3. A multi-layered mirror according to claim 2, wherein said
dielectric multi-layered coating is composed of three dielectric
layers.
4. A multi-layered mirror according to claim 3, wherein said
dielectric multi-layered coating comprises: a plurality of
dielectric layers formed on a front side of said substrate in the
order from the substrate side, of a 2H layer, an L layer and an H
layer.
5. A multi-layered mirror according to claim 3, wherein said
dielectric multi-layered coating comprises: a plurality of
dielectric layers formed on a front side of said substrate in the
order from the substrate side, of an H layer, an L layer and a 2H
layer.
6. A multi-layered mirror according to claim 2, wherein said
dielectric multi-layered coating consists of four dielectric
layers.
7. A multi-layered mirror according to claim 6, wherein said
dielectric multi-layered coating consists of dielectric layers
formed on the front side of the transparent substrate in the order
from the substrate side, of an L layer, a 2H layer, an L layer and
an H layer.
8. A multi-layered mirror according to claim 6, wherein said
dielectric multi-layered coating consists of dielectric layers
formed on the front side of the transparent substrate in the order
from the substrate side, of a 2L layer, a 2H layer, an L layer and
an H layer.
9. A multi-layered mirror according to claim 6, wherein said
dielectric multi-layered coating consists of dielectric layers
formed on the front side of the transparent substrate in the order
from the substrate side, of an L layer, an H layer, an L layer and
a 2H layer.
10. A multi-layered mirror according to claim 2, wherein said
dielectric multi-layered coating consists of five dielectric
layers.
11. A multi-layered mirror according to claim 10, wherein said
dielectric multi-layered coating consists of dielectric layers
formed on the front side of said transparent substrate in the order
from the substrate side, of a 2H layer, an L layer, an H layer, an
L layer and an H layer.
12. A multi-layered mirror according to claim 10, wherein said
dielectric multi-layered coating consists of dielectric layers
formed on the front side of said transparent substrate in the order
from the substrate side, of an H layer, a 2L layer, an H layer, an
L layer and an H layer.
13. A multi-layered mirror according to claim 10, wherein said
dielectric multi-layered coating consists of dielectric layers
formed on the front side of said transparent substrate in the order
from the substrate side, of an H layer, an L layer, an H layer, an
L layer and a 2H layer.
14. A multi-layered mirror according to any one of claims 4, 5 or
7-13, wherein said 2H layer comprises a pair of H layers.
15. A multi-layered mirror according to any one of claims 7 to 9
and 11 to 13, wherein said 2L layer comprises a pair of L
layers.
16. A multi-layered mirror comprising:
a substrate made of one of glass and transparent plastic;
a dielectric multi-layered coating disposed on the front side of
said substrate; and
a light-absorbing layer formed on the backside of said
substrate,
said dielectric multi-layered coating comprising three to six
dielectric layers laminated one on top of another and having
alternately differing refractive indices, one of said dielectric
layers being a single high-refractive index layer and another layer
being a single low-refractive index layer, said high-refractive
index layer and said low-refractive index layer both having an
optical thickness of .lambda..sub.0 /2, where .lambda..sub.0 is a
wavelength of light used as a reference measurement for design
purposes, and each of the remaining layers has an optical thickness
of .lambda..sub.0 /4.
Description
BACKGROUND OF THE INVENTION
The present invention relates to a mirror, and more particularly,
to a multi-layered surface mirror having superior properties in
terms of such aspects as freedom from glare, visibility and
decorative finish.
The multi-layered surface mirror of the present invention is useful
in such applications as automotive rearview mirrors, mirrors with a
convex surface used for warning drivers against hazards in the road
ahead, and ornamental mirrors.
Conventional mirrors such as automotive rearview mirrors are
generally composed of a glass substrate coated on its back side
with a coating of a metal such as aluminum or chromium, which is
further overlaid with a protective coating.
Multi-layered back reflecting mirrors have recently been marketed.
Such mirrors are composed of a glass substrate having a dielectric
multi-layered coating on its back side and which is overlaid with a
black light-absorbing coating. Multi-layered surface mirrors have
also been proposed to be composed of a glass substrate having a
dielectric multi-layered coating on its front side and a black
light-absorbing coating on the back side. In the mirror of the
former type, an example of which is disclosed in Unexamined
Published Japanese Patent Application No. 144504/1982, the
dielectric multi-layered coating in composed of five alternating
films of TiO.sub.2 and SiO.sub.2 that have optical thickness of
.lambda./4 or 3.lambda./4, or a combination of such layer. Another
example of a multi-layered back reflecting mirror is disclosed in
Unexamined Published Japanese Patent Application No. 98405/1985
explaying dielectric multi-layered coating having four alternating
films of TiO.sub.2 and SiO.sub.2, each having an optical thickness
of n.lambda./4 (where n is an odd number). An example of a surface
reflecting mirror of the second type employs a three-layered
dielectric coating of TiO.sub.2 --SiO.sub.2 --TiO.sub.2, each layer
having an optical thickness of .lambda./4. Commercial products of
this type of mirror are believed to have been manufactured in West
Germany.
The above-described prior art mirrors have spectral reflection
characteristics as shown in FIG. 4, in which curve (a) refers to an
aluminum mirror. As curve (a) shows, the aluminum mirror has a high
reflectance (80 to 90%) and flat reflection characteristics which
are not dependent upon wavelength. If this aluminum mirror is used
as a rearview mirror in an automobile, it causes strong reflection
of the light flux from the headlights of a following vehicle, which
leads to eye fatigue because of the glare caused by the reflected
light.
The spectral reflection characteristics of the prior art chromium
mirror are shown by curve (b) in FIG. 4. As curve (b) shows, the
chromium mirror has a low reflectance (38-50%) and exhibits a
certain degree of freedom from glare compared with the higher
reflectance aluminum mirror. However, the glareless property of the
chromium mirror is not as high as it might be expected to be
because of the flat spectral characteristics of the mirror. In
addition, the visibility of the chromium mirror is rather poor.
The dielectric multi-layered back reflecting mirrors disclosed in
Unexamined Published Japanese Patent Application Nos. 144504/1982
and 98405/1985 have spectral reflection characteristics as shown by
curves (c) and (d), respectively, in FIG. 4. These spectral
reflection characteristics are similar to those of a commercial
multi-layered surface reflecting mirror shown by curve (e) in FIG.
4. In general, the visibility of these mirrors and their freedom
from glare are not satisfactory for the same reasons as noted below
for the commercial multi-layered surface reflecting mirror.
Furthermore, these mirrors which are of the back reflecting type,
produce reflection from the front surface of glass and the
resulting image overlap impairs resolution.
A known commercial multi-layered surface reflecting mirror is
composed of a glass substrate that has a three-layered (TiO.sub.2
--SiO.sub.2 --TiO.sub.2) coating former on the front side and a
light-absorbing coating on the back side. The optical thickness of
each layer of the coating is .lambda./4. This three-layered mirror
has spectral reflection characteristics as shown by curve (e) in
FIG. 4. The mirror has a reflectance of 48% and satisfies the
reflectance requirement specified in JIS (Japanese Industrial
Standard) D 5705 "Automotive Mirror System" for a minimum
reflectance of 38% for the chromium mirror. However, as is clear
from curve (e) in FIGS. 4, the reflectance of this mirror peaks in
the range of 430-550 nm and drops sharply in the longer ranges up
to 700 nm, causing the mirror to reflect bluish light. As a result,
the color balance provided by the mirror is disturbed in such a way
that reds are particularly difficult to recognized. Thus, the
visibility of the mirror is impaired.
The spectral luminous efficiency V'(.lambda.) of the human eye, the
spectral energy characteristics P(.lambda.) of a headlight of an
automobile, and the product of P(.lambda.).times.V'(.lambda.) are
shown by three different curves in FIG. 5. As these curves show,
the response of a driver's eyes to the headlights of a closely
following vehicle at night is highest at a wavelength between 480
and 550 nm. Since this range coincides with the region where the
peak of the spectral reflectance curve (e) in FIG. 4 occurs, the
ability of the commercial three-layered mirror and other
conventional surface reflecting mirrors to prevent glare from the
headlights of a following vehicle at night is not as high as
desired.
SUMMARY OF THE INVENTION
An object, therefore, of the present invention is to solve the
aforementioned problems of the prior art mirrors, in particular,
multi-layered surface type mirror, and to provide a multi-layered
mirror having improved properties in such aspects as freedom from
glare, visibility and decorative finish.
The present inventors conducted various studies on the composition
of a dielectric multi-layered coating to be formed on the front
side of a glass or plastic transparent substrate the backside of
which is to be provided with a light-absorbing layer so as to
prepare a multi-layered surface reflecting mirror. The preferred
dielectric multi-layered coating finally produced by the inventors
was composed of 3 to 6 dielectric layers laminated one on top of
another having alternately differing refractive indices, and at
least one of these laminated dielectric layers had an optical
thickness of .lambda..sub.0 /2 (.lambda..sub.0 is the wavelength of
light to be used as the reference for design purposes and is
desirably set in the range of 500 to 580 nm for an incident angle
of 0.degree.), with each of the remaining dielectric layers having
an optical thickness of .lambda..sub.0 /4. The present inventors
found that a dielectric multi-layered coating having this
composition has a greater degree of freedom from glare since the
reflectance in the range of 480 to 550 nm where high spectral
luminous efficiency for dark adaptation occurs was reduced. It was
also found that this dielectric multi-layered coating offered an
improved level of visibility since the reflectances in the ranges
of 430 to 480 nm and 580 to 750 nm for blue and red colors,
respectively, to which the human eye is less sensitive than the
other primary color (green) were increased. The present invention
has been accomplished on the basis of these findings.
The present invention provides a multi-layered mirror having a
dielectric multi-layered coating on the front side of a glass or
plastic transparent substrate and a light-absorbing layer formed on
the back side of the same substrate, the dielectric multi-layered
coating being composed of 3 to 6 dielectric layers laminated one on
top of another having alternately differing refractive indices, at
least one of these dielectric layers having an optical thickness of
.lambda..sub.0 /2 (.lambda..sub.0 is the wavelength of light used
as the reference measurement for design purposes) and each of the
remaining dielectric layers having an optical thickness of
.lambda..sub.0 /4.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1, 2 and 3 are each a set of cross-sectional views showing on
an enlarged scale the essential part of multi-layered surface
reflecting mirrors according to various embodiments of the present
invention;
FIG. 4 is a graph showing the spectral reflection characteristics
of different types of prior art reflector mirrors;
FIG. 5 is a graph showing the spectral energy characteristics of a
headlight of an automobile, the spectral luminous efficiency for
dark adaptation of the human eye, and the product of these two
parameters, namely, the spectral energy characteristics of the
headlight of an automobile as perceived by the human eye; and
FIGS. 6 to 16 are spectral reflection characteristic diagrams for
reflector mirrors according to various embodiments of the present
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention will hereinafter be described in detail with
further reference to the accompanying drawings.
As already discussed, the dielectric multi-layered coating in the
multi-layered mirror of the present invention is composed of 3 to 6
dielectric layers in a laminated relationship having alternately
differing refractive indices. In a particularly preferred
embodiment, the dielectric coating is composed of layers of a
material having a high refractive index (1.9 to 2.4) that alternate
with layers of material having a low refractive index (1.3 to 1.8),
for a total number of layers of 3 to 5. Each of the layers of a
material having a high refractive index is formed of one or more
materials having a high refractive index, and each of the layers of
a material having a low refractive index is also formed of one or
more materials having a low refractive index. Illustrative
materials having a high refractive index include oxides such as
TiO.sub.2, Ta.sub.2 O.sub.5, ZrO.sub.2, CeO.sub.2, HfO.sub.2 and
La.sub.2 O.sub.3, sulfides such as ZnS, and mixtures thereof.
Illustrative materials having a low refractive index include
MgF.sub.2, SiO.sub.2 , CeF.sub.3, and Al.sub.2 O.sub.3 and mixtures
thereof.
Referring to the thickness of the dielectric layers of which the
dielectric multi-layered coating is made, it is essential for the
purposes of the present invention that at least one of the
dielectric layers have an optical thickness of .lambda..sub.0 /2,
with each of the remaining layers having an optical thickness of
.lambda..sub.0 /4. If layers of a material having a high refractive
index alternate with layers of a material having a low refractive
index in such a way that each of the layers has an optical
thickness of .lambda..sub.0 /4, the light reflected from one
interface between layers will interfere with the light reflected
from another interface and the resulting amplification effect
produces spectral reflection characteristics in which a maximum
reflectance occurs at .lambda..sub.0. If, on the other hand,
alternating layers having individual optical thicknesses of
.lambda..sub.0 /4 are interleaved with a layer having an optical
thickness of .lambda..sub.0 /2, spectral reflection characteristics
are attained in which the reflectance at or near .lambda..sub.0 is
comparatively lower than the values in the shorter and longer
wavelength ranges.
During commercial production of the multi-layered mirror of the
present invention, the thickness of the layers in the dielectric
multi-layered coating that are ideally to have an optical thickness
of .lambda..sub.0 /4 should be controlled over the range of
.lambda..sub.0 /4.times.0.8 to .lambda..sub.0 /4.times.1.2, and the
layers that are to ideally have an optical thickness of
.lambda..sub.0 /2 should be controlled over the range of
.lambda..sub.0 /2.times.0.8 to .lambda..sub.0 /2.times.1.2. The
individual dielectric layers may be formed by various methods
including physical coating techniques such as evaporation,
sputtering and ion plating, chemical vapor deposition (CVD)
techniques, and thin-film forming techniques such as one employing
an organic coating solution.
The following are particularly preferred layer compositions for the
multi-layered dielectric coating which is to be formed on the front
side of a transparent substrate in the multi-layered mirror of the
present invention.
(I) When the multi-layered dielectric coating is composed of three
layers:
(i) transparent substrate-2H layer-L layer-H layer
(ii) transparent substrate-H layer-L layer-2H layer
(II) When the multi-layered dielectric coating is composed of four
layers:
(i) transparent substrate-L layer-2H layer-L layer-H layer
(ii) transparent substrate-2L layer-2H layer-L layer-H layer
(iii) transparent substrate-L layer-H layer-L layer-2H layer
(III) When the multi-layered dielectric coating is composed of five
layers:
(i) transparent substrate-2H layer-L layer-H layer-L layer-H
layer
(ii) transparent substrate-H layer-2L layer-H layer-L layer-H
layer
(iii) transparent substrate-H layer-L layer-H layer-L layer-2H
layer.
In (I), (II) and (III) above, the H layer signifies a
high-refractive index film with an optical thickness of
.lambda..sub.0 /4 which is made of a material having a high
refractive index of 1.9 to 2.4, the 2H layer signifies a
high-refractive index film with an optical thickness of
.lambda..sub.0 /2 which is made of a material having a high
refractive index of 1.9 to 2.4, the L layer signifies a
low-refractive index film with an optical thickness of
.lambda..sub.0 /4 which is made of a material having a low
refractive index of 1.3 to 1.8, and the 2L layer signifies a
low-refractive index film with an optical thickness of
.lambda..sub.0 /2 which is made of a material having a low
refractive index of 1.3 to 1.8. The 2H layer may be the combination
of a high-refractive index film (H.sub.1 layer) with an optical
thickness of .lambda..sub.0 /4 made of one high-refractive index
material and another high-refractive index film (H.sub. 2) with an
optical thickness of .lambda..sub.0 /4 that is made of another
high-refractive index material as shown in FIGS. 1a, 1b, 2c and 3a.
In a similar manner, the 2L layer may be the combination of a
low-refractive index film (L.sub.1 layer) with an optical thickness
of .lambda..sub.0 /4 that is made of one low-refractive index
material and another low-refractive index film (L.sub.2 layer) with
an optical thickness of .lambda..sub.0 /4 that is made of another
low-refractive index material as shown in FIG. 3b.
The multi-layered mirror of the present invention employs glass or
plastic as the material of the transparent substrate on which the
dielectric multi-layered coating is formed. Glass is a particularly
preferable substrate material. The light-absorbing layer formed on
the back side of the transparent substrate is preferably of a black
color.
The following examples are given for the purpose of further
illustrating the preferred embodiments of the present invention,
but are in no way to be taken to limit the same.
EXAMPLE 1
FIG. 1(A) is a cross-sectional view showing on an enlarged scale
the essential part of a mirror constructed according to a preferred
embodiment of the present invention in which the dielectric coating
is composed of three layers. In this figure, 1 is a glass
substrate, 2 is a high-refractive index layer (2H layer) made of
TiO.sub.2 and which has an optical thickness of .lambda..sub.0 /2
(270 nm), 3 is a low-refractive index layer (L layer) made of
MgF.sub.2 and which has an optical thickness of .lambda..sub.0 /4
(140 nm), 4 is a high-refractive index layer (H layer) made of
TiO.sub.2 and which has an optical thickness of .lambda..sub.0 /4
(140 nm), and 9 is a black light-absorbing layer.
Thus, the multi-layered mirror of Example 1 has a dielectric
multi-layered coating formed on the front side of the glass
substrate composed, in order from the substrate side, of a 2H
layer, an L layer and an H layer, with the black light-absorbing
layer being coated on the back side of the glass substrate. The
spectral reflection characteristics of the resulting multi-layered
mirror are shown in FIG. 6, from which one can see the following:
Compared with the conventional multi-layered mirror, the mirror of
Example 1 has a greater degree of freedom from glare since the
reflectance in the range of 480 to 550 nm, having a high value of
the product of spectral luminous efficiency for dark adaptation and
the spectral energy of a headlight, is reduced. Secondly, the
visibility of the inventive mirror is increased since the
reflectances in the ranges of 430 to 480 nm and 580 to 700 nm for
blue and red, respectively, for which the human eye is less
sensitive than for green, are increased.
EXAMPLE 2
A mirror of the type shown in FIG. 1(B) was fabricated by forming a
light-absorbing layer 19 on the back side of a glass substrate 11
and a three-layered dielectric coating on the front side of the
substrate. The dielectric coating was composed, in order from the
substrate side, of an H layer 12 (TiO.sub.2, .lambda..sub.0 /4=135
nm), an L layer 13 (MgF.sub.2, .lambda..sub.0 /4=135 nm) and an
H.sub.1 +H.sub.2 combination layer 14 (H.sub.1 =TiO.sub.2,
.lambda..sub.0 /4=135 nm; H.sub.2 =La.sub.2 O.sub.3, .lambda..sub.0
/4=135 nm) equivalent to a 2H layer (.lambda..sub.0 /2). The
spectral reflection characteristics of this mirror are shown in
FIG. 7, from which one can see the great freedom from glare of the
mirror of Example 2, as well as its high visibility.
EXAMPLE 3
A mirror of the type shown in FIG. 2(A) was fabricated by forming a
light-absorbing layer 29 on the back side of a glass substrate 21
and a four-layered dielectric coating on the front side of the
substrate. The dielectric coating was composed, in order from the
substrate side, of an L layer 22 (SiO.sub.2, .lambda..sub.0 /4=135
nm), a 2H layer 23 (TiO.sub.2, .lambda..sub.0 /2=270 nm), an L
layer 24 (MgF.sub.2, .lambda..sub.0 /4=135 nm) and an H layer 25
(TiO.sub.2, .lambda..sub.0 /4=135 nm). The spectral reflection
characteristics of this mirror are shown in FIG. 8, from which one
can see the great freedom from glare of the mirror of Example 3, as
well as its high visibility.
EXAMPLE 4
A mirror of the type shown in FIG. 2(B) was fabricated by forming a
light-absorbing layer 39 on the back side of a glass substrate 31
and a four-layered dielectric coating on the front side of the
substrate. The dielectric coating was composed, in order from the
substrate side, of a 2L layer 32 (SiO.sub.2, .lambda..sub.0 /2=270
nm), a 2H layer 33 (TiO.sub.2, .lambda..sub.0 /2=270 nm), an L
layer 34 (MgF.sub.2, .lambda..sub.0 /4=135 nm) and an H layer 35
(TiO.sub.2, .lambda..sub.0 /4=135 nm). The spectral reflection
characteristics of this mirror are shown in FIG. 9, from which one
can see the great freedom from glare of the mirror of Example 4, as
well as its high visibility.
EXAMPLE 5
A mirror of the type shown in FIG. 2(C) was fabricated by forming a
light-absorbing layer 49 on the back side of a glass substrate 41
and a four-layered dielectric coating on the front side of the
substrate. The dielectric coating was composed, in order from the
substrate side, of an L layer 42 (MgF.sub.2, .lambda..sub.0 /4=135
nm), an H layer 43 (TiO.sub.2, .lambda..sub.0 /4=135 nm), an L
layer 44 (MgF.sub.2, .lambda..sub.0 /4=135 nm), and an H.sub.1
+H.sub.2 combination layer 45 (H.sub.1 =TiO.sub.2, .lambda..sub.0
/4=135 nm; H.sub.2 =La.sub.2 O.sub.3, .lambda..sub.0 /4=135 nm)
equivalent to a 2H layer (.lambda..sub.0 /2). The spectral
reflection characteristics of this mirror are shown in FIG. 10,
from which one can see the great freedom from glare of the mirror
of Example 5, as well as its high visibility.
EXAMPLE 6
A mirror of the type shown in FIG. 3(A) was fabricated by forming a
light-absorbing layer 59 on the back side of a glass substrate 51
and a five-layered dielectric coating on the front side of the
substrate. The dielectric coating was composed, in order from the
substrate side, of an H.sub.1 +H.sub.2 combination layer 52
(H.sub.1 =TiO.sub.2, .lambda..sub.0 /4=135 nm; H.sub.2 =La.sub.2
O.sub.3, .lambda..sub.0 /4=135 nm) equivalent to a 2H layer
(.lambda..sub.0 /2), an L layer 53 (SiO.sub.2, .lambda..sub.0
/4=135 nm), an H layer 54 (TiO.sub.2, .lambda..sub.0 /4=135 nm), L
layer 55 (SiO.sub.2, .lambda..sub.0 /4=135 nm) and an H layer 56
(TiO.sub.2, .lambda..sub.0 /4=135 nm). The spectral reflection
characteristics of this mirror are shown in FIG. 11, from which one
can see the great freedom from glare of the mirror of Example 6, as
well as its high visibility.
EXAMPLE 7
A mirror of the type shown in FIG. 3(A) was fabricated by forming a
light-absorbing layer 59 on the back side of a glass substrate 51
and a five-layered dielectric coating on the front side of the
substrate. The dielectric coating was composed, in order from the
substrate side, of an H.sub.1 +H.sub.2 combination layer 52
(H.sub.1 =TiO.sub.2, .lambda..sub.0 /4=135 nm; H.sub.2 =La.sub.2
O.sub.3, .lambda..sub.0 /4=135 nm) equivalent to a 2H layer
(.lambda..sub.0 /2), an L layer 53 (SiO.sub.2, .lambda..sub.0
/4=135 nm), an H layer 54 (ZrO.sub.2, .lambda..sub.0 /4=135 nm), L
layer 55 (SiO.sub.2, .lambda..sub.0 /4=135 nm) and an H layer 56
(TiO.sub.2, .lambda..sub.0 /4=135 nm). The spectral reflection
characteristics of this mirror are shown in FIG. 12, from which one
can see the great freedom from glare of the mirror of Example 7, as
well as its high visibility.
EXAMPLE 8
A mirror of the type shown in FIG. 3(B) was fabricated by forming a
light-absorbing layer 69 on the back side of a glass substrate 61
and a five-layered dielectric coating on the front side of the
substrate. The dielectric coating was composed, in order from the
substrate side, of an H layer 62 (TiO.sub.2, .lambda..sub.0 /4=135
nm), an L.sub.1 +L.sub.2 combination layer 63 (L.sub.1 =Al.sub.2
O.sub.3, .lambda..sub.0 /4=135 nm; L.sub.2 =SiO.sub.2,
.lambda..sub.0 /4=135 nm) equivalent to a 2L layer (.lambda..sub.0
/2), an H layer 64 (TiO.sub.2, .lambda..sub.0 /4=135 nm), an L
layer 65 (SiO.sub.2, .lambda..sub.0 /4=135 nm), and an H layer 66
(TiO.sub.2, .lambda..sub.0 /4=135 nm). The spectral reflection
characteristics of this mirror are shown in FIG. 13, from which one
can see the great freedom from glare of the mirror of Example 8, as
well as its high visibility.
EXAMPLE 9
A mirror of the type shown in FIG. 3(C) was fabricated by forming a
light-absorbing layer 79 on the back side of a glass substrate 71
and a five-layered dielectric coating on the front side of the
substrate. The dielectric coating was composed, in order from the
substrate side, of an H layer 72 (TiO.sub.2, .lambda..sub.0 /4=135
nm), an L layer 73 (SiO.sub.2, .lambda..sub.0 /4=135 nm), an H
layer 74 (TiO.sub.2, .lambda..sub.0 /4=135 nm), an L layer 75
(SiO.sub.2, .lambda..sub.0 /4=135 nm), and an H.sub.1 +H.sub.2
combination layer 76 (H.sub.1 =TiO.sub.2, .lambda..sub.0 /4=135 nm;
H.sub.2 =ZrO.sub.2, .lambda..sub.0 /4=135 nm) equivalent to a 2H
layer (.lambda..sub.0 /2). The spectral reflection characteristics
of this mirror are shown in FIG. 14, from which one can see the
great freedom from glare of the mirror of Example 9, as well as its
high visibility.
EXAMPLE 10
A mirror of the type shown in FIG. 3(D) was fabricated by forming a
light-absorbing layer 89 on the back side of a glass substrate 81
and a five-layered dielectric coating on the front side of the
substrate. The dielectric coating was composed, in order from the
substrate side, of an H layer 82 (TiO.sub.2, .lambda..sub.0 /4=135
nm), an L layer 83 (SiO.sub.2, .lambda..sub.0 /4=135 nm), an H
layer 84 (TiO.sub.2, .lambda..sub.0 /4=135 nm), an L layer 85
(SiO.sub.2, .lambda..sub.0 /4=135 nm), and a 2H layer 86
(TiO.sub.2, .lambda..sub.0 /2=270 nm). The spectral reflection
characteristics of this mirror are shown in FIG. 15 from which one
can see the great freedom from glare of the mirror of Example 10,
as well as its high visibility.
The multi-layered surface reflecting mirror of the present
invention has the following technical advantages.
(i) The mirror has a great degree of freedom from glare.
In the case of driving at night with the light of the headlamps of
a following vehicle striking the mirror, the quantity of light flux
reaching the driver's eyes after being reflected by the mirror is
obtained by multiplying the intensity (power) of the light P
(.lambda.) (shown by curve (2) in FIG. 5) times the eye's response
to power V'(.lambda.) (spectral luminous efficiency for dark
adaptation) (shown by curve (1) in FIG. 5, the product of these two
parameters, P(.lambda.).times.V'(.lambda.), is plotted by curve (3)
in FIG. 5) further times the spectral reflectance of the mirror. As
is clearly evident from FIG. 5, in order to attain a greater degree
of freedom from glare, it is important that the reflectance of the
mirror have a small value in the wavelength region where
P(.lambda.).times.V'(.lambda.) assumes a high value. As can be seen
from FIGS. 6 to 15 the mirror of the present invention has a low
reflectance in the range of 480 to 550 nm where
P(.lambda.).times.V'(.lambda.) takes on a large value, which
provides the mirror with a greater degree of freedom from
glare.
(ii) The mirror has a high visibility level.
As described above in (i), the reflectance of the mirror may be
reduced in order to provide it with a greater degree of freedom
from glare, but this results in the mirror having a lower
visibility level. The visibility of a mirror increases as its
reflectance increases. As FIGS. 6 to 15 show, the mirror of the
present invention has a low reflectance in the range of wavelengths
where a greater degree of freedom from glare is ensured, but in the
other wavelength range, it has a sufficiently high reflectance to
assure a superior visibility level.
In addition to these favorable reflectance properties, the mirror
of the present invention is characterized by good color contrast,
which contributes to a further improvement in the visibility
offered by the mirror. The three primary colors, blue, green and
red, have wavelengths of 400 to 480 nm, 490 to 580 nm and 590 to
700 nm, respectively. In terms of brightness, the visual sensation
(stimulus to the eye) is small in the blue and red regions, as is
clear from curve (1) in FIG. 5. The mirror of the present invention
is designed so that it has a lower reflectance in the green region
(where a stronger visual sensation occurs in terms of brightness)
than in the blue and red regions, thereby "sensitizing" these
low-brightness colors to provide an improved color contrast, and
hence a further increase in visibility level.
(iii) The mirror is also improved in terms of aesthetic
aspects.
As a result of the aforementioned improvements in the degree of
freedom from glare and the level of visibility, the reflector
mirror of the present invention produces reflected light having a
somewhat magenta hue. This magenta hue imparts a perception of high
quality to the mirror, and there enabling it to be clearly
differentiated from other mirrors on the market.
(iv) The mirror blocks deleterious wavelengths of light.
FIGS. 6 to 15 show the spectral reflection characteristics of the
mirror of the present invention in the wavelength range of 300 to
750 nm. The spectral reflection characteristics of the same mirror
over the wavelength range extended to 1300 nm are shown in FIG. 16,
from which one can see that the mirror of the present invention has
the additional advantage of inhibiting and blocking the reflection
of unwanted and deleterious light such as ultraviolet and infrared
radiation.
* * * * *